Hydraulic accumulator assemblies



Mill'eh 2 1965 R. LEDUc HYDRAULIC AccuMuLAToR AssEMBLIEs original Filed July e, 1960 4 Sheets-Sheet l.

March 2, 1965 R. LEDuc 3,171,254

HYDRAULIC ACCUMULATOR ASSEMBLIES Original Filed July 6, 1960 4 Sheets-Sheet 2 March 2, 1965 R. LEDuc 3,171,254

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United States Patent O TLZSe HYDRAULEC AECUMULATR ASSElt/illi Ren Leduc, 23 Rue Henri loppet, lle Vesmet, Seine-et-Gise, France @riginal application July 6, 1960, Ser. No. 41,657, now llatent No. itlf, dated Feb. il, 1964. Brvided and this application Get. 22, 1963, Ser. No. Sltjdlel Claims priority, application France Fully '7, i953, 'W95-ttl l'7 tCl. eil-S1) This application is a division of application Serial No. 41,057, tiled July 6, 1960, and entitled Double-Acting Fluid-Pressure lack, which issued on February ll, 1964, as U.S. Patent No. 3,120,788.

This invention relates to fluid-pressure jacks and their controls; also to the combination of a huid-pressure jack with a pump-fed duid-pressure accumulator.

An object of the invention is a double-acting fluidpressure jack, which may be of the differential piston type, in which once a movement of the piston in either direction has been started by the appropriate control action, the piston moves briskly and continues to move until its stroke has been completed, even if the controlling member is further displaced (so long as the displacement of the controlling member is not excessive).

A more particular object of the invention is a jack as above described, the piston of which has feed and exhaust ducts in it terminating in ports which can be opened and closed by means carried by a control rod axially slidable in the piston and mechanically unconnected therewith, the arrangement being such that displacement of the control rod in either direction from a neutral range of position, whose extent may approxnnate to zero, and in which the feed and exhaust ports are both closed, causes the piston to move in the opposite direction.

To ensure that the stroke of the jack piston, when once started, is completed without ail (provided the control rod is not grossly mishandled) a further object of the invention is a jack as above described, in which the means carried by the control rod for bringing about opening and closing oi the feed and exhaust ports each have an axial extension at least equal to the full stroke of the piston; and preferably, but not necessarily, so constructed that the travel of the control rod, relatively to the piston, in either direction from the neutral range, is at least equal to the full stroke of the piston.

With an arrangement as above described, movement of the piston (in either direction) once started by a displacement, from the neutral position range, of the control rod in the opposite direction, proceeds at an accelerated rate until the piston-stroke is completed, unless the control rod is displaced, relatively to the piston, in the reverse direction, i.e. the same direction as the movement of the piston, through a distance sufficient to bring it back into the neutral position range, which, it is to be remembered, is a range of position relative to the piston; so that, in effect, to stop the piston before it has completed its stroke, the control rod must be displaced, in the same direction as the movement of the piston, quickly enough to catch the piston up.

When a huid-pressure jack as above described, more particularly a hydraulic jack, is combined with a pumpfed hydraulic pressure accumulator, a still further object of the invention is an arrangement, in which the accumulator pressure is applied to a movable member, in opposition to the etfort of resilient means, such as a spring or a compressed gas, which member is operatively connected to the control rod of the jack, and which includes a relief valve, which when open by-passes the pump-delivery direct to exhaust, such relief valve being so controlled by the jack piston that an increase of accumulator presice sure displaces the control rod in the direction to cause the jack piston to execute its stroke (in the opposite direction) and thereby open the relief valve, and conversely, in such a way that if the accumulator pressure exceeds a predetermined, upper, critical limit, the pressure is relieved, and if it falls below another predetermined lower critical limit, the accumulator is replenished by the pump. The utility of this feature is hereinafter explained.

Such a combined arrangement may also include a nonreturn valve ina connection between the pump and the liquid chamber of the accumulator, operative to prevent back-:dow from the accumulator towards the pump.

It may also include means, carried by the control rod of the jack, operative to open both the feed and exhaust ports of the ducts within the jack piston, should the accumulator pressure exceed a predetermined, high, critical limit, as might occur as a result of thermal expansion of the hydraulic liquid.

These various objects of the invention willv be more fully explained hereinafter, in the non-limitative description, with reference to the accompanying drawings, of certain embodiments of the invention.

The invention has numerous practical applications.

(a) For operating a circuit-breaker in an electrical transmission network, especially a high tension network: in this case, the circuit must be broken quickly and with complete certainty when a prescribed limiting tension (voltage) is exceeded, and it is essential that a circuitbreaking action, once started, shall not Vbe interrupted, even if the tension immediately falls below the critical limit: a hydraulic (or fluid-pressure) control system according to this invention ensures this; Vand moreover ensures an automatic and positive reconnection of the circuit when the tension has fallen below a prescribed, lower, critical limit;

(b) For operating gates of hydraulic and the like power systems, especially in hydro-electric installations. In such cases, a common requirement is that the gates should open completely when a certain maximum upstream water-level is reached, and close completely when this water-level is lowered below a certain minimum: a heat-controlled system according to this invention will satisfy these requirements.

Modern hydraulic circuits are usually of the closedcenter type. Many of them comprise a feed-pump, a reservoir or sump from which the pump draws the liquid (and which receives the liquid exhausted from the service circuitland an accumulator serving as a reservoir of energy. To avoid useless consumption of energy, and incidentally thereby to increase the life of the pump, it is desirable to return the pump delivery directly to the reservoir with the least possible loss of head, when the accumulator is full or has attained the design pressure. lt is likewise desirable to re-establish the normal circuit, i.e. to me the pump deliver to the accumulator, when the accumulator volume has decreased to a prescribed extent or its pressure has fallen to a prescribed, lower, critical limit.

These requirements are met by the relief valve feature of the invention, previously mentioned.

The objects and features of the invention already mentioned and how they may be put into practice, will be better understood from consideration of the following description, having reference to the accompanying drawings, of several embodiments of the invention, given by way of example only and without implied limitation of the scope of the invention, as defined in the hereto appended claims.

ln the drawings:

FIGURES 1 to 3 are schematic views, in axial section of three elementary embodiments of the invention;

FIGURES 4 to 7 are axial sections of a practical embodiment of the jack and its control of the invention, intended for combination with a pump-fed pressure accumulator and including a relief valve, respectively showing four different settings of the asser Ably;

FIGURES 8 to ll are similar views of an alternative embodiment, including the accumulator, which in this instance is unitary with the jack and its control means;

and

FIGURES l2V to l5 are similar views of another alternative embodiment, similar in principle to that of FlGURES 8 to l1, but incorporating means for simultaneously opening the feed and exhaust ports of the jack piston, when the accumulator pressure exceeds a prescribed upper critical limit` In what follows left and right are to be read as seen-in the ligure unless-this is repugnant to the context.

T he embodiment of FIGURE l is a double-acting jack with differential piston. It comprises a cylinder 3, a piston I whose right hand face is of smaller area than the left hand face, and a control rod l@ axially displaceable with respect to the piston. The-right hand chamber ofthe cylinder is permanently connected to the external feed line 2. ln the piston are formed a feed duct 4 and an exhaust duct 6 communicating with an external exhaust connection at 7. The feed and exhaust ducts terminate respectively in feed and exhaust ports formed in an axial `bore of the piston in which slide enlargements or lands 8, 9 formed on the control rod lll. In the piston is also formed a duct leading from the left hand face ofthe piston to a port situated half-way between the feed and exhaust ports. f

Each of the'lan'ds tl, 9 has an axial extension greater thanthe full stroke of the piston'inthe cylinder, and Ythe i distance separating these lands is slightly less than that separating the feed and'exhaust ports, so that when the control rod is in neutral position (or range of position,

. whose extent in the` illustrated lexample is almost nil) with respect to the piston, both the feedand exhaust ports are covered by the lands S, 9 as shown in the ligure. The possible travel of the control rod, relatively to the piston,

Sa, 8b, respectively, and those communicating with the right hand cylinder chamber by ball-valves 9a, 9b, re-` spectively. These valves are arranged in the piston 1 and the feed valves tia, 9a communicate, via an annular clearance I3 around the piston, with the external feed 2, while the exhaust valves 8b, 9b communicate, via a longitudinal duct in the piston with the external exhaust connection 7. Ealls 3a, Sb are respectively unseatable by cam-faced enlargements Ella, 1lb, of the control rod l@ and balls 9a, 9b by similar enlargements i251, 12b, each of which is longer than the full stroke of the piston. In the ligure the control rod is in the neutral position, all the ball-valves being closed. It will be evident from inspection of the iigure that leftward movement of the control rod will unseat balls Sez and 9b and connect the left hand cylinder chamber to the external feed and the right hand cylinder chamber to exhaust, causing the piston to move to the right and complete itsk stroke, rightward movement of the control rod producing the contrary result.

FIGURES 4 to 7 illustrate the application of a jack device as illustrated in FIGURE '2 to the automatic control o the charging of a pump-fed hydraulic pressure accumulator. rl`he pump (not shown) feeds the accumulator through a non-return valve (not shown). The device comprises a body 2li, in which is formexl the jack cylinder'i, in which slides the piston 23, the latter having a central bore in which sliA s the control rod 27 having cam-faced enlargements 33, 75 controlling the feed and exhaust connections of the left cylinder chamber as in FIGURE 2. Elements 26,

2S, Z7, 33, 35, 36 of FIGURES 4 to 7 correspond respectively to elements 3, l, lt), l2, ll, 9a and tia of FlGURE 2;

The body 29 also encloses a chamber 2l communicat- Y by a duct ZZ'with the left hand chamber of cylinder 26 and with an external feed line connected to the liquid chamber of the accumulator. Chamber Z1 lis of variable volume being closed by a sliding partition 23 resiliently in either direction from this neutral position, isgreater than the-full stroke of the piston.

If the control rod is displaced tothe left from the' position shown, the feed port (of duct 4) will be uncovered by land 8 and the exhaust portV (of Vduct 6) will remain covered by land 9; Fluid under the feed pressureY will thus be delivered, via ducts Y4 and 5, to the left hand Vchamber ofthe cylinder and since the area of the left hand face `of the piston is greater than that of its right hand face the piston will be driven to the' right and this movement will accelerate until the feed port of duct 4 is fully uncovered and will continue until the piston has completed its stroke. To reverse the stroke of the piston the control rod must be movedY to the right relatively to the piston until it has passed through the neutral position shown inthe4 ligurev and the land 8 has covered the feed port (4) and uncovered the exhaust port (e) thus disconnecting the left hand cylinder chamber from the external feed 2 and connecting said chamber to theY external exhaust connection '7. f k

' Since the left hand end of the b ore of piston 1 is blind, it is connected to exhaust by an extension of the exhaust duct 6 to prevent hydraulic lock.

In FIGURE 2, the lands 8, 9 of FIGURE 1 are replaced by cam-faced enlargements of the control rod, which as before are each longer` than the full stroke of the piston and these enlargements are operative to unseat balls Sa, 9a, which, when seated, close the feed and exhaust ducts 4,V 6, respectively. Otherwise the embodiment of FIG- URE 2 operates in the same way as that of FIGURE l.

' In the embodiment of FIGURE 3, the two faces of piston 1 are of equal area and the feed and exhaust ports areduplicated, .those communicating with the left hand chamber of the cylinder 3 being controlled by ball-valves loaded by a spring 241 in opposition to the accumulator pressure inchamber 2l. Partition 23 is rigidly connected to the control rod 'Z7'. The body 2t) also has internal ducts 39, 3l, respectively connectedV to the pump delivery and to the accumulator as indicated Vby arrows F1, F2 and a by-pass duct 32 connected to exhaust into which theV exhaust duct 37 (corresponding to duct 6 of FIGURE l) of the jack piston 2S delivers.

|Ehe ducts Si) and Si communicate with the by-pass duct 3f?. through an opening on which a spring-loaded ball 2:9 is seatable'(see FIGURES 4, 7). Ball 29 is unseatable by an extension of the jack piston 2S when the latter completes its leftward'stroke (see FiGURES 5, 6). In this arrangement, the range of neutral travel of the control rod 27 is of quite appreciable extent.

This arrangement operates as follows: with the jac piston 28 in the position of FlGURE 4, the ball 29 is seated and the pump charges the accumulator through ducts Tail, 3ft (arrows Fl, F2). As the accumulator pressure rises the partition 2.3 moves to the right compressing spring 25. In FIGURE 4 the pressure has nearly reached the prescribed maximum limit and the control rod has nearly reached one limit of its neutral range. In FIG- URE 5,'the accumulator pressure has reached the prescribed maximum limit and the further rightward movement of partition '23 and control rod .2F/has unseated the ball 34, thus allowing the left hand cylinder chamber to be exhausted and causing the piston ZS to complete itsY stroke to the left and unseat ball Z9, thus by-passing the left and when the pressure has nearly fallen to a lowerV prescribed critical limit the control rod has nearly reachedl the other limit of its neutral range as shown in FIG- URE 6. When this lower limit of pressure is attained for unseating balls 34, 36-

the leftward movement of the partition 23 and rod 27 has unseated the ball 36, the ball 34 being already seated, so that the left hand cylinder chamber is connected to the duct 22, which is at accumulator pressure, and disconnected from exhaust, so that the piston 28 is moved to the right through its full stroke and allows the ball 29 to be re-seated by its spring as shown in FIGURE 7. The pump delivery is now no longer by-passed to exhaust but feeds the accumulator once more and builds up pressure therein until the situation shown in FIGURE 4 is reached, thus initiating another cycle of movements.

The embodiment of FIGURES 8 to 11 incorporates the accumulator itself, but is otherwise similar to that of FIGURES 4 to 7. FIGURES 8 to 11 correspond respectively to FIGURES 4 to 7 and show corresponding positions of the parts. The elements numbered Ztl, 2?., 27, 2S, 29, 30, 32, 37 in FIGURES 4 to 7 are designated in FIGURES 8 to 11 by 4G, 51, 45, 46, 50, 47, 49, 52, respectively.

In FIGURES 8 to il, the variable volume liquid chamber 4I of the accumulator is within the body 40 and takes the place of the chamber 21 of FIGURES 4 to 7. It is separated by the movable partition 42 (corresponding to partition 23 of FIGURES 4 to 7) from a chamber 43 containing compressed gas, which constitutes the resilient means opposing the liquid pressure in chamber 41. The latter is fed from the pump (not shown) by a continuation of duct 47 containing a spring-loaded non-return valve 48, which prevents back-flow from the accumulator towards the pump. The external connection of duct 51, subjected to accumulator pressure, leads to the service line of the accumulator. In this case the movable partition 42 is connected to the control rod 45 by a coupling 44, 53.

The arrangement operates in much the same way as that of FIGURES 4 to 7, which has already been described, the only diterence being that the action of the jack-piston 46 is determined by the volume of the chamber 41, rather than the pressure therein. This is because the spring 24 of FIGURES 4 to 7 exerts a substantially constant effort, the travel of partition 23 of FIG- URES 4 to 7 being relatively small, whereas the effort exerted by the gas pressure in chamber 43 of FIGURES 8 to 11 is substantially inversely proportional to the volume of chamber 43.

The embodiment of FIGURES l2 to 15 is similar in principle to that of FIGURES 4 to 7, reference characters common to both sets of figures designating corresponding elements.

In FIGURES 12 to 15, the seatable element of the by-pass control valve 29 is constituted by the right hand end of the jack-piston 23 which is loaded by a spring 62 in a closed chamber 63, which spring serves only to overcome frictional effects, when the accumulator pressure is nil or very low. The movable member defining the variable volume of the chamber 2 is the left hand end et? of the control rod 27; and since the uid pressure counteracted by the spring 24 is transmitted through the control rod 27, the latter is not rigidly connected to the abutment of spring 24, as in FIGURES 4 to 7, but has a push-only connection therewith.

The embodiment of FIGURES 12 to 15 also includes a non-return valve 61, corresponding to the non-return valve 48 of FIGURES 8 to 11, operative between the pump-delivery duct Sti (arrow F1) and the accumulator feed duct 3i (arrow F2) which has a branch 31a leading to the service line (arrow F4) of the accumulator. The exhaust duct 37 is constituted by the annular clearance between the control rod 27 and the extension 29 of the jack-piston 28 constituting the by-pass valve for the pumpdelivery; and this clearance communicates, via the chamber 2S containing spring 24 with exhaust outlet duct 32 (arrow F3).

In the embodiment of FIGURES 12 to l5, the neutral range of travel of the control rod, relatively to the Y v ti jack-piston, is of negligible extent (see FIGURE 13, in which the position of the several elements corresponds to that of FIGURE 4). The position shown in FIG- URE 12 corresponds to that of FIGURE 7, it being appreciated that in the embodiment of FIGURES 12 to 15 the smaller area face of the jack-piston is the left hand face, not the right hand face as in FIGURES 4 to 7. The position shown in FIGURE 14 corresponds to that shown in FIGURE 5.

The embodiment of FIGURES 12 to 15 also includes an additional safety device for relieving excess accumulator pressure, due for instance to overheating.

Normally, yafter the ball A, corresponding to ball 36 of FIGURES 4 to 7, has been unseated to apply the accumulator pressure to the larger area (right hand) face of the jack-piston 28 as shown in FIGURE 14, the control rod 27 will be displaced leftwards as the accumulator pressure falls, ultimately causing ball A to be reseated and the ball B, corresponding to ball 34 of FIG- URES 4 to 7, to be unseated, thus bringing about an abrupt return to the position of FIGURE 12.

However, if the accumulator pressure increases, e.g. due to overheating, this increased pressure acting on the end 69 of the control rod, will cause a continued displacement of the control rod 2'7 to the right, Ia shown in FIGURE 15.

`The additional safety device consists essentially in providing, on the control rod 27, a further cam-faced enlargement 64 facing the opposite Way to the cam-face of the enlargement which normally unseats ball B, so positioned that, when the control rod is displaced rightwards from the position shown in FIGURE 15, both the balls A and B are unseated, thus connecting the accumulator directly with the exhaust outlet.

Such an excess of pressure fed to the system could also occur if the jack-piston 28 were to jam in the position shown in FIGURE 13.

It will be understood that the invention is not limited to the embodiments hereinbefore particularly described with reference to the accompanying drawings, but includes all such modifications, variations and adaptations as are Within the competence of those skilled in the art and are within the scope and spirit of the invention as dened in the hereto appended claims.

I claim:

1. In combination with a Huid-pressure jack which includes a cylinder, a piston movable in said cylinder and having formed therein a feed duct and an exhaust duct, respectively communicating with a feed port and an exhaust port, a control rod coaxially slidable in said piston and mechanically unconnected therewith, and means carried by said rod for bringing about opening and closure of said feed and exhaust ports respectively, said feed and exhaust ports being so positioned that movement of said controlrrod in either direction from a neutral range of position, the extent of which may approximate to zero, in which both the feed and exhaust ports are closed, brings about a movement of the piston in the opposite directions, a liquid-pressure accumulator, a feed duct therefor, a nonreturn valve in said duct, an exhaust duct, a relief valve controlling communication between said accumulator feed duct and said exhaust duct, means actuated by the piston of the fluid-pressure jack for opening saidV relief valve, a movable member subjected to the accumulator pressure, and resilient means acting on said movable member in opposition to the accumulator pressure, said movable member being operatively connected to the control rod of the duid-pressure jack to move the control rod to cause the jack piston to open the relief valve when the accumulator pressure exceeds a predetermined upper critical limit and to bring about closure of the relief valve when the accumulator pressure falls below a predetermined lower critical limit.

2. The combination defined in claim 1 in which the said resilient means is constituted by a spring.

3; The Vcombination deinedV in claim 1, in which the said resilient means isconstituted by a compressed gas.

4. The combination deiined in claim '1, includingl a direct feed-connection from the liquid chamber of the accumulator to the cylinder of the duid-pressure jack.

5. The combination defined in claim 1, in which the said movable member is the movable partition member enclosing the variable-volume liquid chamber of the accumulator itself, the liquid in said chamber being kept under pressure by the said resilient means operative on said partition member. Y

6. The combination defined in claim 1, further including means carried by the control rod and operative to open the feed and exhaust ports of the duid-pressure jack piston simultaneously, when the accumulator pressure exceeds a predetermined critical limit.

7. In a device for supplying iluid under pressure, in combination, accumulator pressureY chamber means defined in part by a movable member which moves to a mini'num volume position in a Volume-decreasing direction decreasing the volume of said chamber means during a drop in the pressure of a iluid therein and which moves to a maximum-volume.position in an opposite volume-increasing direction increasing the volume of said chamber means during an increase in the pressure of a duid therein; pump conduit means communicating with said chamber means for supplying fluid under pressure thereto; low pressure conduit means communicating with said pump conduit means Vto receive uid under pressure therefrom so as to prevent the fluid under pressure idowing to said chamber means; valve means between said pump conduit means and low pressure conduit means for directing fluid under pressure from said pump conduit means to said pressure chamber meansV when said valve means is closedl and for directing uid under pressure from said pump conduit means to said low pressure conduitV means when said valve means isV open; a cylinder; piston means shiftable -in said cylinder between an opening end position and a closed end position and cooperating with said valve means for opening the latter when saidpiston -means isV in said opening end position and for closing said valve means when said pistonfmeans isin said Vclosing end position; and control means operatively connectedrto said-movable member and piston means for automatically controlling fluid in said cylinder to displace said pistonV means from said opening to said closing eind position thereof when said movable member reaches said minimum-volume position and to displace said piston means from said closing-to said opening end position when said movable member reaches said maximum-volume position.

8. In a device as recited in claim 7, yieldable means Y acting on said movable member for urging the latter in said volume-decreasing direction and opposing the movement thereof in said volume-increasing direction.

9. In a device as recited in claim `8, said yieldable means being in the form of a chamber containing compressed-gas.

10. In a device as recited in claim S, said yieldable means being in the form of a spring acting on said movable member.

11. In a device as recited in claim 8, said piston means being hollowand said control means including an elongated control rod shiftable in said hollow piston means, said accumulator pressure'chamber means forming an extension of said cylinder and said movable member being formed by an end of said control rod; said yieldable means acting on an opposite end of said control rod.

` 12. In a device as recited in claimll, said valve means being formed by an end of said piston means and said control means placingpsaid accumulator pressure chamber means directly in communication With said low pressure conduit means when the pressure in said pressure chamber means increases to` displace said movable member beyond said maximum volume position thereof.

13. A device for supplying iiuid under pressure, comprising, in combination, a cylinder; elongated hollow piston means slidable in said cylinder and deiining therewith a small-area chamber on one side of said piston means and a large-area chamber on the other side of said piston means so that when both chambers have liquid at the same pressure the total pressure of the large-area chamber will be greater than that of the smallarea chamber to advance the piston means in said cylinder to an end position where said small-area chamber has a minimum volume and said large-area chamber has a maximum volume; conduit means communicating with said small-area chamber for maintaining tluid under pressure therein; pressure-passage means formed in said piston means and placing said chambers in communication with each other soV that when said pressure passage means is open the iiuid under pressure -will tlow from said small-area chamber to said large-area chamber to displace said piston means to said end position thereof; low pressure passage means formed in said piston means and placing only said large-area chamber in communication with said low-pressure passage means when the latter is open so that said piston means will then be displaced to an opposite end position where the small-area chamber has its maximum volume and said large-area chamber has its minimum volume; a pair of valve means respectively cooperating with said pair of passage means for opening and closing the same; elongated control rod means axially shiftable in said piston means for opening one ot said valve means when said control rod means shifts in one direction relative to said piston means and for opening the other of said valve means when said control rod means shifts in an opposite direction relative to said Vpiston means so that the displacement of said piston means between said end positions thereof is controlled by the direction of movement of said control rod means relative to said piston means; ,accumulator pressure chamber means in which accumulator fluid under pressure is located, said accumulator pressure chamber means being formed in part by a movable member which moves to change the volume of said accumulator pressure chamber means; yieldable pressure means acting on said movable member for displacing the latter in a direction decreasing the volume of saidacctunulator pressure chamber means as the pressure in the latter decreases and said yieldable pressure means yielding during an increase in the pressure in said accumulator pressure chamber means so that said movable member then moves in a direction increasing the volume of said accumulator pressure chamber means; connecting means connecting said control rod means to said movable member for moving said control rod means in one direction upon movement ofV said movable member in one direction and for moving said control rod means in an opposite direction upon movement of said movable member in an opposite direction so that when the volume of said accumulator pressure chamber reaches a given minimum value said piston means will be automatically displaced to one of its end positions and when said volume of said accumulator pressure chamber reaches a given maximum value said piston means will be automatically displaced to its other end position; pump conduit means for guiding pressure fluid discharged 4from a pump; delivery conduit means for delivering pressure tiuid from said pump conduit means to said accumulator pressureV position determined by the maximum volume of said accumulator pressure chamber and closing said third valve means when displaced to its end position corresponding -to the minimum volume of the accumulator pressure chamber, so that the pressure in the latter will be automatically maintained between given limits.

14. lIn a device as recited in claim 13, said accumulator pressure chamber means being in .the form of an elongated cylinder and said movable member being in the form of a partition slidable in said cylinder and said yieldable pressure means being located in the latter cylinder on one side of said partition -for urging Ithe latter to move in a direction which decreases the volume of said accumulator pressure chamber means.

15. A device recited as in claim 14 and wherein said yieldable pressure means is in the form of a compressed gas.

16. A device as recited in claim 14 and wherein said yieldable pressure means is Iin the form of a spring.

17. A device as recited in claim 13 and wherein said connecting means connects said movable member to an end of said control rod means for movement with the latter, said yieldable pressure means engaging an opposite end of said control rod means to act on said movable member through said control rod means and said accumulator pressure chamber `forming, an extension of said cylinder.

References Cited by the Examiner UNITED STATES PATENTS 2,956,405 1.0/60` Spalding et al 60-51 3,116,607 1/64 Gratzmuller 60--51 FOREIGN PATENTS 551,847 3/ 43 Great Britain.

JULIUS E. WEST, Primary Examiner. 

1. IN COMBINATION WITH A FLUID-PRESSURE JACK WHICH INCLUDES A CYLINDER, A PISTON MOVABLE IN SAID CYLINDER AND HAVING FORMED THEREIN A FEED DUCT AND AN EXHAUST DUCT, RESPECTIVELY COMMUNICATING WITH A FEED PORT AND AN EXHAUST PORT, A CONTROL ROD COAXIALLY SLIDABLE IN SAID PISTON AND MECHANICALLY UNCONNECTED THEREWITH, AND MEANS CARRIED BY SAID ROD FOR BRINGING ABOUT OPENING AND CLOSURE OF SAID FEED AND EXHAUST PORTS RESPECTIVELY, SAID FEED AND EXHAUST PORTS BEING SO POSITIONED THAT MOVEMENT OF SAID CONTROL ROD IN EITHER DIRECTION FROM A NEUTRAL RANGE OF POSITION, THE EXTENT OF WHICH MAY APPROXIMATE TO ZERO, IN WHICH BOTH THE FEED AND EXHAUST PORTS ARE CLOSED, BRINGS ABOUT A MOVEMENT OF THE PISTON IN THE OPPOSITE DIRECTIONS, A LIQUID-PRESSURE ACCUMULATOR, A FEED DUCT THEREFOR, A NONRETURN VALVE IN SAID DUCT, AN EXHAUST DUCT, A RELIEF VALVE CONTROLLING COMMUNICATION BETWEEN SAID ACCUMULATOR FEED DUCT AND SAID EXHAUST DUCT, MEANS ACTUATED BY THE PISTON OF THE FLUID-PRESSURE JACK FOR OPENING SAID RELIEF VALVE, A MOVABLE MEMBRE SUBJECTED TO THE ACCUMULATOR PRESSURE, AND RESILIENT MEANS ACTING ON SAID MOVABLE MEMBER IN OPPOSITION TO THE ACCUMULATOR PRESSURE, SAID MOVABLE MEMBER BEING OPERATIVELY CONNECTED TO THE CONTROL ROD OF THE FLUID-PRESSURE JACK TO MOVE THE CONTROL ROD TO CAUSE THE JACK PISTON TO OPEN THE RELIEF VALVE WHEN THE ACCUMULATOR PRESSURE EXCEEDS A PREDETERMINED UPPER CRITICAL LIMIT AND TO BRING ABOUT CLOSURE OF THE RELIEF VALVE WHEN THE ACCUMULATOR PRESSURE FALLS BELOW A PREDETERMINED LOWER CRITICAL LIMIT. 